That's what u was thinking when first found it, didn't have the texture of most the sandstone around here and below is a dark really different hard layer .
Your probably correct though just wanted to see if was something else...
could be dirty sandstone, mixed with sediment or leached into by other crud. That hardened inner might just be iron concretions embedded within, sandstone is famous for attracting Fe. Try a magnet see if it has a weak attraction